1. Field of the Invention
The present invention relates to novel weakly acidic colloidal dispersions of a cerium (IV) compound in an aqueous medium, and to a process for the preparation of such weakly acidic colloidal dispersions of a cerium (IV) compound.
In the description which follows, a colloidal dispersion of a cerium (IV) compound in an aqueous medium is also designated by the term "sol".
2. Description of the Prior Art
Colloidal dispersions of a cerium (IV) compound are known to this art; they are useful for a wide variety of applications. Moreover, one major application for these dispersions is in heterogeneous catalysis and, in particular, for the catalytic conversion of exhaust gases emanating from internal combustion engines, also termed the catalysis of automotive post-combustion reactions.
Indeed, cerium (IV) is an important promoter of such catalytic activity, in particular because of its capacity for storing oxygen and thus creating an oxygen supersaturation in the vicinity of the catalytically active elements such as platinum, palladium or the like.
For this purpose, the colloidal dispersions are used as a medium for impregnating a support with cerium, with cerium oxide being produced by calcination of the impregnated support and, thus, by decomposition of the cerium (IV) compound contained in the dispersion.
Moreover, the acid nature of the colloidal dispersion influences the support-impregnating process, which must be suited to the characteristics of said support and in particular to its surface properties.
Thus, colloidal dispersions of a cerium (IV) compound, notably a cerium hydroxynitrate, are known to this art and are very highly acidic, typically having a pH of less than 1. These dispersions present numerous problems during impregnation, when the support to be impregnated is slightly basic in nature.
Published European Patent Application No. 316,205, on the other hand, describes the production of such colloidal dispersions which are weakly acidic in nature, generally having a pH higher than 3.5.
This process entails destabilizing an aqueous colloidal dispersion of a cerium hydroxynitrate by an acid having a pKa lower than that of nitric acid, for example by acetic acid.
Such process is also complex, because it comprises the successive production of two colloidal dispersions, a first dispersion by subjecting a hydroxide to attack by nitric acid, followed by a second dispersion by destabilization of the first suspension using acetic acid. Moreover, it requires a significant consumption of reagents such as nitric acid and acetic acid.